Black Holes May Get Supermassive by Eating Stars

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The supermassive black holes thought to lurk at the heart of most
galaxies attained their enormous sizes by eating stars, a new
study suggests.

Some theories hold that these galactic
black holes — which seem to contain millions to billions
of times the mass of our sun — get so hefty by consuming huge
quantities of gas or merging with other black holes; others
suggest the objects were simply born large. But the new study
reports that they likely get supermassive mainly by ripping apart
double-star systems and devouring one of the stars.

"I believe this has got to be the dominant method for growing
supermassive black holes,"
lead author Benjamin Bromley of the University of Utah said in a
statement.

The team's work follows up on the 2005 discovery of hypervelocity
stars, which were flung out of our Milky Way's center by
gravitational forces and are traveling at speeds of 1.5 million
mph (2.4 million kph) or more — fast enough to escape the galaxy
and go zooming off into the depths of space.

Hypervelocity stars are thought to originate in binary systems
that wander too close to the Milky Way's central black hole,
which appears to weigh as much as 4.3 million suns. Tidal forces
eject one star, the theory goes, and capture the other, which
eventually becomes food for the growing black hole. [ Photos: Black
Holes of the Universe ]

The researchers modeled each step in this process, and their
results pinpointed stars as the galactic monsters' chief
sustenance.

"We put the numbers together for observed hypervelocity stars and
other evidence, and found that the rate of binary encounters
[with the Milky Way's supermassive black hole] would mean most of
the mass of the galaxy's black hole came from binary stars,"
Bromley said. "We estimated these interactions for supermassive
black holes in other galaxies and found that they too can grow to
billions of solar masses in this way."

As many as half of all stars are in binary pairs, so they are
plentiful in the Milky
Way and other galaxies, researchers said.

The results further indicated that our galaxy's supermassive
black hole has likely doubled to quadrupled in mass during the
past 5 billion to 10 billion years by eating stars.

"When we look at observations of how stars are accumulating in
our galactic center, it's clear that much of the mass of the
black hole likely came from binary stars that were torn apart,"
Bromley said.

The scientists published their results online Monday (April 2) in
The Astrophysical Journal Letters.